Recently, intelligent reflecting surface (IRS) [1]} and its variants, e.g., reconfigurable intelligent surface (RIS) [2]}, [3]}, large intelligent surface/antenna (LISA) [4]}, [5]}, have arisen as a promising technology to build the future spectral- and energy-efficient wireless networks in a cost-efficient manner [6]}, [7]}. Specifically, IRS is a programmable metasurface consisted of a huge number of passive reflecting elements, each of which can alter the incident signals by imposing an independent magnitude and/or phase shift [1]}, [9]}. As a result, the propagation environment between transmitters and receivers can be customized according to the specific quality-of-service (QoS) requirements, which brings new degree-of-freedom for enhancing the wireless channel capacity and mitigating the inter-channel interference [10]}. By contrast to the conventional cooperative communications with active relays, the IRS elements passively reflect the incident signals without requiring any radio-frequency (RF) chains [11]}, [12]}. The recollection of the diffused energy in the air makes IRS an excellent option for green communications [13]}, [14]}. Besides, IRS is capable of operating in the full-duplex (FD) mode without incurring the self-interference as in the traditional FD relay [1]}, [16]}. Moreover, the low-cost material and patch structure render the IRS can be easily attached to the environment objects, e.g., the walls, ceilings, building surfaces [10]}, [18]}. Thanks to these potential benefits, it is expected to deploy massive IRSs in wireless networks to facilitate the future 6G ecosystem [19]}, [7]}, [21]}.
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